The answer is that at some point the font for this post was changed and a number of special symbols all got changed to ?. Fortunately, a very kind person on the stackexchange mathematica forum was able to dig out the original post from the internet archive:
https://web.archive.org/web/20170102024443/https://community.wolfram.com/groups/-/m/t/987658
which does work - so relieved.
